This invention relates to containers and more particularly to thermoplastic resin-made containers for food service.
Hitherto, containers made of various different types of material have been developed for food service. For example, in order to provide good heat insulation, containers are formed of a foamed polystyrene sheet material. Such container is most typically used as a handy container for instant food service or the like.
With all its good heat insulation performance, however, such polystyrene sheet-made container does not well withstand heating in a microwave oven and will be easily deformed in a short time when subjected to such heating. As such, it can hardly be qualified as a heat-resisting container for use under microwave heating.
For containers other than those of the foamed polystyrene sheet type, research and development efforts have been made in an attempt to provide them with good heat resistance. In the present state of the art, there are available heat-resisting containers formed of non-foamed sheet materials and which are claimed to withstand microwave oven heat. However, these containers have no sufficient heat insulation or heat retention performance; when removed from the oven, some of them may feel too hot, and some other is such that its contents may get cold in a very short time. As such, from the standpoint of serviceability, they are not well suitable for use with microwave ovens.
In view of such situations, the present inventor made an attempt to form a container by laminating a polycarbonate resin film or sheet on the interior-side surface of a foamed polystyrene sheet in order to provide the container with heat resisting properties while best utilizing the heat insulating properties of the foamed polystyrene sheet. It was found that while such lamination was possible, the sheets laminated together could not be formed into a container (by a sheet forming technique such as vacuum forming or air-pressure forming) because of there being a great difference in temperature resistance between foamed polystyrene and polycarbonate resin. So, this attempt was unsuccessful after all.
After further efforts directed toward the development of a container having good serviceability under microwave heat conditions, the inventor came up with an idea that it might be possible to form a container from a laminated-sheet prepared so that a polyethylene terephthalate film or sheet is present on the interior surface of a foamed polystyrene sheet, when the container is formed. However, if a prestretched polyethylene terephthalate film or sheet of ordinary type is laminated on a foamed polystyrene sheet, there is a difficulty that when the laminated sheet is formed into a container, the greater the draw ratio, the greater would be the stretching due to drawing of the peripheral wall portion, with the result of the thickness of some peripheral wall portion being reduced disproportionally to the other wall portion. It follows that the container would have a portion excessively weakened due to such wall thickness reduction. Further, it was found that if subjected to microwave heat conditions, such container would be liable to considerable deformation because the prestretched polyethylene terephthalate film or sheet would tend to return to its original state, which fact practically denies the serviceability of the container for use with a microwave oven.